There is a magnetic card reader system formed of a magnetic head and a computer connected with the magnetic head (see Patent Document 1). The magnetic head is formed of a head main body that reads data stored in a magnetic card and a control unit that converts an analog signal read by the head main body into a digital signal and encrypts the digital signal by using a one-way function. The head main body and the control unit are accommodated in a head container. The control unit of the magnetic head uses a key stored in an RAM thereof to encrypt a digital signal, and transmits the encrypted digital signal to the computer. A control unit of the computer uses a key stored therein to decrypt the encrypted digital signal.
In this system, when the control unit of the magnetic head transmits the encrypted digital signal to the control unit of the computer, the control unit of the computer instructs the control unit of the magnetic head to change the key. A key changing procedure in this system is as follows. When the control unit of the computer decrypts the digital signal received from the magnetic head, it newly generates a key and transmits the generated key to the control unit of the magnetic head. The control unit of the magnetic head replaces the existing key stored in the RAM with the newly transmitted key. Further, when an operator inputs a function changing instruction and a new function from a keyboard, the control unit of the computer transmits the function changing instruction and the new key to the control unit of the magnetic head. The control unit of the magnetic head replaces an existing function with the newly transmitted function.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-143213